Neutron generators

ABSTRACT

In a sealed-off neutron generator containing tritium, for the production of neutrons by the D-T reaction, helium-3 gas accumulates owing to radioactive decay of the tritium and interferes with the proper operation of the generator. In the present generator a sputter-ion pump is sealed to the generator to remove the helium-3 from the envelope prior to admitting hydrogen isotope gas from the usual replenisher before use. The pump is adapted to remove only the unwanted helium-3 and not the wanted D and T, by having its sputterable cathodes made of, or coated with, a metal which has a low chemical affinity for hydrogen but is readily sputtered by helium, such as copper.

United States Patent [191 Stark I NEUTRON GENERATORS [75] Inventor:Donald Sutherland Stark, Baldock,

England [73] Assignee: Nationale Research Development Corporation,London, England [22] Filed: Dec. 18, 1970 [2]] Appl. No: 99,331

[ Feb. 26, 1974 [5 7 ABSTRACT ln a sealed-off neutron generatorcontaining tritium, for the production of neutrons by the D-T reaction,helium-3 gas accumulates owing to radioactive decay of the tritium andinterferes with the proper operation of the generator. In the presentgenerator a sputter-ion pump is sealed to the generator to remove thehelium- 3 from the envelope prior to admitting hydrogen isotope gas fromthe usual replenisher before use. The pump is adapted to remove only theunwanted helium- 3 and not the wanted D and T, by having its sputterablecathodes made of, or coated with, a metal which has a low chemicalaffinity for hydrogen but is readily sputtered by helium, such ascopper.

8 Claims, 1 Drawing Figure [30] Foreign Application Priority Data Dec.30, 1969 Great Britain 62570/69 [52] US. Cl. 313/615, 33 1 /7 [51] Int.Cl. H01j 39/00 [58] Field of Search 313/61 R, 61 S [56] References CitedUNITED STATES PATENTS 2,964,665 12/1960 Brinkerhoff et al. 313/61PATENTED FEB26 I374 NEUTRON GENERATORS BACKGROUND OF THE INVENTION Thisinvention relates to neutron generators and relates particularly togenerators comprising a sealed-offv envelope or tube in which deuteriumand/or tritium ions from an ion source are accelerated to strike atarget containing deuterium and/or tritium to produce neutrons by theD-D and/or D-T reactions. A generator of this type is described, forexample, in U.K. Specification No. 1,088,088 and corresponding US. Pat.No. 3,448,314.

In the latter type of sealed-off generator, a low pressure (e.g., about0.02 mTorr) of deuterium and/ortritium is present throughout the tubeduring operation. This is obtained from a gas replenisher containing ahydrogen-occluding metal (such as titanium) in which deuterium and/ortritium gas has been previously absorbed. The desired pressure in thetube is maintained by controlling the temperature of the replenisher.When the replenisher is switched off the gas is reabsorbed by thereplenisher and the pressure should fall to approximately zero.

In generators which contain tritium, for producing neutrons by the D-Treaction, a problem arises from the fact that tritium decaysradioactively (with a halflife of about l2.3 years) to helium-3 (He-3).Thus a continuously increasing concentration of He-3 atoms builds up inall parts of the tube where tritium is held, (for example the target,gas replenisher and ion source walls into which tritium is driven by theradiofrequency discharge which, as in the aforementioned generator, isfrequently used to ionize the gas).

Since He-3 is a chemically inert atom, it is not chemically bonded tothe target, replenisher and walls in the same way as its parent tritiumatom. There is thus a gradual release of l-Ie-3 atoms, which build up agas pressure in the free space within the tube.

This free He-3 gas can cause reduction of neutron output in severalways:

i. The He-3 gas within the ion source is ionized so that the pressurewithin the ion source is incorrect and the extracted deuterium and/ortritium ions are contaminated with He-3 ions.

ii. The deuterium and/or tritium ions, during their acceleration towardsthe target, undergo various electronic interactions with the neutrall-Ie-3 atoms, which, in general, reduce their velocity of impact on thetarget.

ii Amssssixq prevent the striking of an ionizing discharge source.

iv. The voltage which can be applied between electrodes withoutbreakdown is reduced.

It is an object of the present invention to provide a neutron generatorin which the problem of l-Ie-3 accumulation is alleviated.

SUMMARY OF THE INVENTION According to the present invention, in asealed-off neutron generator which contains tritium and which includes areplenisher for providing hydrogen isotope gas, there is provided a pumpin continuous communication with the generator envelope for removinghelium from the envelope, said pump being adapted to pump gas from theenvelope when the pump is oper- B2,;tnrsssurstnlbqiqnso e can in thibriated, and substantially not to absorb hydrogen gas from the envelopewhen the pump is not operated.

The pump is preferably a sputter-ion pump having sputterable electrodemeans whereof the sputterable metal has a low chemical affinity forhydrogen.

Preferably the sputterable metal has a high sputtering coefficient forhelium ions.

The sputter-ion pump is preferably of the triode type, the sputterableelectrode means comprising at least one multi-apertured, eg grid ormesh, cathode located between an anode and a further electrode on towhich, in operation, metal is sputtered from the cathode. The pump maybe external to the envelope and permanently sealed thereto.

Sputterable metals suitable for the sputterable electrode includecopper, silver and gold. The sputterable metal may be a deposit on astronger metal such as stainless steel, monel or molybdenum, eg, platedon to a grid or mesh of such metals.

The sputter-ion pump serves to remove the accumulated gaseous He-3periodically. Before switching on the hydrogen-isotope replenisher, thepressure in the tube is checked. A reading other than zero indicates thepresence of He-3 gas and the I-Ie-3 pump is oper ated until a zeroindication is obtained. The pump is then switched off, and thereplenisher operated to give the working hydrogen isotope pressure.

As will be hereinafter explained, conventional sputter-ion pumps areunsuitable for use in the present invention because such pumps comprisemetals, such as titanium, which have a high chemical affinity forhydrogen and which would therefore remove the wanted D and T as well asthe unwanted He-3.

DESCRIPTION OF THE DRAWING To enable the nature of the present inventionto be more readily understood, attention is directed, by way of example,to the accompanying drawing, which is a sectional elevation of part of aneutron generator tube embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The drawing shows the ion-sourceend of a neutron generator tube of the kind described in U.K.Specification No. 1,088,088. It comprises a glass tube or envelope 1surrounded by a radiofrequency exciting winding 2. Within the tubeanaluminum-coated copper electron-stopper 3 is supported on a metal tube4. At-

tached to the tube 1 are a gas replenisher 5, a sealingoff tube 6 and aPirani pressure gauge, the latter being hidden behind sealing-off tube6.

The envelope 1 includes a metal portion 7 to which, in accordance withthe present invention, there is sealed a triode-type of sputter-ion pump8 (sometimes called a getter-ion pump). Pump 8 comprises an envelope 9of non-magnetic stainless steel surrounded by hollow cylindricalpermanent magnet 10 to provide an axial magnetic field. Within theenvelope 9 are mounted a cylindrical anode 11, a pair of mesh cathodes12 and a pair of further elecrodes formed as enddiscs 13. The anode andthe end-discs are earthed to the envelope 9 and are also of stainlesssteel. The mesh cathodes l2 and 12 are mounted on a lead-throughconnection 14 sealed to an insulating bush 15, and in use are connectedto a negative potential, suitably about 4.5 kV.

In the present example the upper mesh cathode 12 is made of stainlesssteel wire 0.0148 inch in diameter with wires per inch, and is platedwith copper to a thickness of 0.002 inch. The lower mesh cathode 12' ismade of copper wire 0.0124 inch in diameter with 24 wires per inch. Theuse of plated stainless steel wire for cathode 12 prevents it distortingunder the weight of cathode 12', which is suspended from it by thenarrow flat strips 16. Anode 11 and discs 13 are mounted on envelope 9by further narrow flat strips (omitted for clarity) in a conventionalmanner.

The operation of a conventional sputter-ion source will be familiar tothose skilled in the vacuum art, but briefly electrons emitted from themesh cathodes are accelerated towards the anode and oscillate to and frobetween the two cathodes before eventually arriving at the anode. Theelectron path length is increased by spiralling in the magnetic field.The electrons produce ionization of the gas molecules by collision, andthe ionized gas molecules are accelerated away from the anode. Somestrike the mesh cathodes and cause sputtering of metal from the cathodeon to the end-discs. Other ions, together with excited atoms, passthrough the cathode meshes and reach the end-discs with just enoughenergy to cause their sorption. These loosely held ions and atoms arethen sealed into the end-discs by burial under further sputtered metalfrom the cathodes, and are thus permanently pumped. In conventionalsputter-ion sources the sputterable mesh cathode is usually made oftitanium.

The above type of pumping action takes place for both inert gases likeHe-3 and for chemically active gases like hydrogen and its isotopessince it depends only on ionization and electronic excitation processes.However, for chemically active gases there is an additional, much fasterprocess by which pumping takes place. This is due to the chemicalaffinity between the gas and the titanium film continuously sputtered onto the end-discs. Neutral molecules of hydrogen, for example, strike theend-discs by normal gas-kinetic processes, and being chemically active,they have a finite probability of becoming bonded to the titanium filmand hence pumped.

In the present invention it is essential that the pump should not pumpor absorb gaseous D or T which are present during operation of thegenerator. Otherwise (a) it would tend to empty the gas replenisher, and(b) its pumping action for He-3 would be impaired by saturation effects.It has been found that such a conventional sputter-ion pump willcontinue to absorb hydrogen even when the cathode voltage is switchedoff. This is because, once the titanium film has been deposited, itschemical sorption action does not depend on the presence of electricalvoltages.

In accordance with the present invention the hydrogen absorption problemis alleviated by making the cathode meshes of a metal which does notreadily react chemically with hydrogen. Hence the film sputtered overthe end-discs is relatively chemically inactive and the pumping actionsubstantially ceases immediately the voltage applied to the cathodes isswitched off. Preferably the cathode meshes are made of a metal having ahigh sputtering coefficient for helium ions. In this way, maximum I-Ie-3pumping speeds can be obtained and, because of the deeper burial of ionsand excited atoms, the re-emission of pumped He-3 is minimised. Examplesof high-sputtering metals which do not readily react with hydrogen arecopper, silver and gold. As meshes made of such metals tend to softenwhen the generator is vacuum-baked during manufacture, these metals canadvantageously be deposited, e.g., by plating, on meshes made ofstronger metals such as stainless steel, monel and molybdenum.

As will be known, there exists a diode type of sputterion or getter-ionpump whose pumping mechanism is similar to that of the triode type,except that ion pumping takes place at the same electrodes as are beingsputtered. This can result in a re-emission of gas already pumped, whichis particularly serious for inert gases, which are not chemicallypumped. It is therefore preferred to use the triode type of pump in thepresent invention, where the pumping and retention of inert He-3 are therequirements.

The accompanying drawing is approximately to scale, the externaldiameter of envelope 9 at its widest being about 1 inch.

The suitability of the above-described pump for the present requirementis indicated by the following typical performance data, using an axialmagnetic field of about 900 oersteds and about 4 kV applied to thecathodes. V

i. The pump operates satisfactorily at 180 mTorr helium pressure.

ii. lt pumps down from this pressure to zero in approximately 7 hoursand from 50 mTorr in 1 /2 hours, in a volume of about 1 litre.

iii. It pumps 1600 litre mTorr of helium without reduction of pumpingspeed and with no indication of saturation.

iv. After pumping 1600 litre.mTorr of helium, the pump releases heliumgas at the low rate of 6 litre.m- Torr in the first week and at 3.5litre.mTorr per week thereafter.

v. Despite the cathode sputtering which accompanies the pumping of 1600litre.mTorr of helium, the pump does not absorb hydrogen gas at ameasurable rate when the cathode voltage is switched off thereafter.

I claim:

1. A sealed-off neutron generator which contains tritium and whichincludes a replenisher for providing hydrogen isotope gas, wherein thereis provided a pump in continuous communication with the generatorenvelope for removing free helium gas from the envelope, said pump beingadapted to pump gas from the envelope when the pump is operated, andsubstantially not to absorb hydrogen gas from the envelope when the pumpis not operated.

2. A neutron generator as claimed in claim 1 wherein said pump is asputter-ion pump having sputterable electrode means whereof thesputterable metal has a low chemical affinity for hydrogen.

3. A neutron generator as claimed in claim 2 wherein the sputterablemetal has a high sputtering coefficient for helium ions.

4. A neutron generator as claimed in claim 3 wherein the sputter-ionpump is of the triode type, the sputterable electrode means comprisingat least one multiapertured cathode located between an anode and afurther electrode on to which, in operation, metal is sputtered from thecathode.

5. A neutron generator as claimed in claim 3 wherein the sputterablemetal is selected from copper, silver and gold.

3 7 94,8 7 5 5 6 6. A neutron generator as claimed in claim 3 wherein 8.A method of operating a neutron generator as the sputterable metal is adeposit on a stronger metal. Claimed in claim 1 comprising operatingsaid pump to 7 A neutron generator as claimed in claim 1 wherein removehelium from the envelope before admitting hy' the pump is external tosaid envelope and sealed 5 drogen isotope gas from replemsherthereto.

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,794,875 Dated Feb. 26, 1974 Inventor(s) Donald Sutherland Stark It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the heading, the priority date is December 23, 1969.

In the heading, the assignee is correctly spelled National.

Signed and sealed this 30th dayof Jilly 1971 (SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 6037 6-P69 FORM po-wse (10-69) a i u.s. eovzmmzu-rmam-nus omcs Ian o-sss-zm,

, UNITED STATES PA-TENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,794,875 Dated Feb. 26, 1974 I e fl Donald Sutherland Stark It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the heading, the priority date is December 23, 1969.

In the heading, the assignee is correctly spelled National.

Signed and sealed this 30th dayofJi ly 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-1 (10-69) uscoMM oc wan-p09 v I k U.S, GOVERNMENTPRINTING OFFICE 2 l9, 0*365'334,

1. A sealed-off neutron generator which contains tritium and whichincludes a replenisher for providing hydrogen isotope gas, wherein thereis provided a pump in continuous communication with the generatorenvelope for removing free helium gas from the envelope, said pump beingadapted to pump gas from the envelope when the pump is operated, andsubstantially not to absorb hydrogen gas from the envelope when the pumpis not operated.
 2. A neutron generator as claimed in claim 1 whereinsaid pump is a sputter-ion pump having sputterable electrode meanswhereof the sputterable metal has a low chemical affinity for hydrogen.3. A neutron generator as claimed in claim 2 wherein the sputterablemetal has a high sputtering coefficient for helium ions.
 4. A neutrongenerator as claimed in claim 3 wherein the sputter-ion pump is of thetriode type, the sputterable electrode means comprising at least onemulti-apertured cathode located between an anode and a further electrodeon to which, in operation, metal is sputtered from the cathode.
 5. Aneutron generator as claimed in claim 3 wherein the sputterable metal isselected from copper, silver and gold.
 6. A neutron generator as claimedin claim 3 wherein the sputterable metal is a deposit on a strongermetal.
 7. A neutron generator as claimed in claim 1 wherein the pump isexternal to said envelope and sealed thereto.
 8. A method of operating aneutron generator as claimed in claim 1 comprising operating said pumpto remove helium from the envelope before admitting hydrogen isotope gasfrom the replenisher.